Confused? Me too, so I have been researching. While in spoken English we tend to use the two interchangeably, in written English, well, it’s complicated. The rule is that feet is the plural of foot, so if we write “the frost giant is fifteen feet tall.” That’s easy. But… when used as a hyphenated adjective, we write “the fifteen-foot-tall frost giant.”

Why? I don’t know.

PS — fire giants can grow to thirty feet in height or more. Just saying.